M12.4.1, an m IgM B cell lymphoma of strain BALB/c mice was drug marked, and B cell hybridomas obtained by somatic cell fusion of this cell line with normal B-lymphocytes from strains C57BL/6J, A/J and the F1 (BALB/c x A/J). All such hybridomas bear H-2 and Ia antigens from the normal cells on their membranes; all express m IgM. The hybridomas could be induced to secrete IgM, and in one case IgG2a, by treatment in vitro with anti IgM. Lipopolysaccharide from E. coli, and T cell factors generated in vitro were less efficient inducers.